Rediscovering American roots music can be incredibly rewarding, but it is not without its pitfalls. Many artists end up sounding like cover bands, for one thing. Not so for the Gamble Brothers. For the past five years, this four-piece Beale Street fixture has been pushing the envelope on the very styles the modern curators are trying to recreate. But while their sound is evolving, it still strongly reflects all the southern rock, Stax and Muscle Shoals soul and R&B, and New Orleans funk they grew up with. Recently they’ve been grooving across the country promoting their third album, Continuator.